Purification of benzoic acid



United States Patent C) No Drawing. Filed Dec. 3, 1962, Ser. No. 242,869 8 Claims. (Cl. 260-525) This invention relates to a method for purifying crude aromatic carboxylic acids obtained as the product of catalytic partial oxidation of an alkylated aromatic hydrocarbon. It relates particularly to the purification of benzoic acid made by such a process.

The general procedure whereby a lower alkylated benzeneis partially oxidized by an oxygen-containing gas to the corresponding aromatic carboxylic acid and this crude acid is further reacted to make a phenol is now well known and is commercially applied. The oxidation of the aromatic hydrocarbon is commonly done in the presence of a metallic catalyst which includes cobalt as an active component. The product stream from such oxidation of toluene, for example, consists of benzoic acid, unreacted toluene, and catalyst, together with small amounts of by-products such as tars, hydrocarbons other than toluene, and various oxygenated compounds. After a preliminary purification to remove the bulk of the unoxidized toluene, catalyst residues, and oxidation byproducts, a crude benzoic acid product is obtained which then may be used as the feed stock for a process for converting aromatic carboxylic acids to the corresponding phenols. I is frequently desirable to diversify the production of phenol plants using such a process by separating and further purifying a portion of the crude benzoic acid feed stream to a technical or higher grade of purity and selling it as such.

Conventional methods for purifying benzoic acid include distillation and sublimation. Recrystallization of the crude material from hot water is also a well established method for obtaining highly purified benzoic acid. However, the nature of the impurities contained in the particular crude benzoic acid obtained from the oxidation process described above makes these conventional methods of purification relatively ineffective. The impurities include, for example, benzaldehyde, benzyl alcohol, benzyl esters, and related compounds which have vapor pressures or solubilities close to those of the desired benzoic acid and are consequently difiicult to remove from the product in substantial entirety by any of the known procedures.

It has now been found that portions of a crude benzoic acid produced by incomplete oxidation of toluene with molecular oxygen can be separated and benzoic acid of particularly high purity can be obtained by a partial extraction process wherein the molten crude benzoic acid is contacted with liquid water at a temperature between about 95 C. and about 117 C., the water-crude benzoic acid proportion being such that two liquid aqueous benzoic acid phases coexist in substantial equilibrium.

The phase diagram for the system benzoic acid-water is well known, for example, see Findlay, The Phase Rule and Its Applications, page 175, 9th edition (1951), Dover Publications. This diagram shows the area on a plot of composition against temperature within which the system exists as two liquid phases in equilibrium, the heavy or organic phase being a solution of water in benzoic acid and the light or aqueous phase being a solution of benzoic acid in water. This area has temperature limits of about 94.6 C., the invariant point, and about l17.2 C., the critical solution temperature. The composition limits are about 6% to about 73% by weight of benzoic acid. It is within this two-phase area that the present process is operated.

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Under these conditions, Where there are present two aqueous benzoic acid phases in equilibrium, the oxidation by-products in the crude benzoic acid would be expected to distribute themselves between the two liquid phases according to their relative solubilities in water. To the contrary, however, it has been found that these impurities are almost completely excluded from the light phase and are found essentially entirely in the organic phase even through some of these by-products have water solubilities as great as or greater than benzoic acid itself. The aqueous phase obtained under these conditions is substantially a solution of pure benzoic acid and when it is separated and cooled, crystalline benzoic acid of exceptional purity is precipitated. Color and odor, as well as impurities, are also largely removed by this purification process and the benzoic acid product is essentially colorless and odorless.

Prior art processes employing water extraction of somewhat similar mixtures teach superior results when using superheated water under high pressure. Such processes necessarily require relatively large heat input and the use of pressure equipment, resulting in higher costs and increased complexity of operation. In the present process it has been discovered that best results are obtained by running the extraction step at 100 C., below the atmospheric boiling point of water, thereby making available all the advantages of operation under atmospheric or near-atmospheric pressure. When operating under these conditions, about 0.6-14 parts by weight of water per part of crude benzoic acid are employed, preferably about 0.77 parts of water. In selecting the proportion of water to be used within the above limits, the desirability of extracting a larger amount of benzoic acid with a relatively high proportion of water must be balanced against the smaller yield of higher purity benzoic acid obtained with a lower water ratio. Surprisingly, there is no build-up of impurity concentration in the filtrate obtained on separation of crystallized benzoic acid from the cooled aqueous extract, and the aqueous filtrate can be recycled to the extraction process without harmful effect on product quality. This is in contrast to conventional recrystallization of benzoic acid wherein the crude acid is completely dissolved in water and the impurities remain dissolved in the aqueous filtrate from the cooled solution. The organic layer from the extraction may be returned to the crude benzoic acid stream being used as feed for a phenol plant or other process or it may be partially recycled to the extraction process.

Alternatively,'this process may be operated as a twostage separation from a superheated water extraction. This mode of operation is based on the known fact that mixtures of certain proportions of benzoic acid and water exist in two liquid phases between 94.6 C., the invariant point, and 1l7.2 C., the critical solution temperature. Crude benzoic acid is contacted with about O.7-7 parts by weight of water, preferably 0.8-5 parts, at about 117 C. under conditions such that substantial equilibrium is attained and two liquid phases are present. Superatmospheric pressure at least sufficient to maintain the water in the liquid state is necessary during this step. The lighter or aqueous layer is separated and cooled to about 95-100 C., whereupon this solution separates into a second aqueous layer and a second organic layer. Cooling the separated second aqueous layer to a temperature below 95 C., preferably to about 1050 C., causes crystallization therefrom of essentially pure benzoic acid, substantially free of the original impurities. Under preferred operating conditions, this product when filtered and dried is of sufiicient purity to also of better than 99 percent purity, thereby qualifying as a good technical grade of benzoic acid, and needs only separation, cooling, and drying for sale as such.

The first organic layer obtained in this mode of op- As shown by the above data, SIIIQHGIfPQOPOIiIlOQS of water yield, from the light phase, c rystalline benzoic acid of significantly higher purity. Increasing the water ratio, while increasing correspondingly the quantity of eration also retains nearly all of the original impurities, benzoic acid extracted, produces acid from the light even though some of these impurities are as soluble in phase of lower quality. When the water is increased to water as benzoic acid itself. This organic layer may be a point where all the benzoic acid is dissolved and no returned to the crude benzoic acid stream for conversion heavy phase is present, i.e., a conventional recrystallizato phenol or it may be recycled in part to the purification, the recrystallized acid obtained on cooling the solution process and contacted again with superheated water. tion is of even P q y- Therefore, in this way there are obtained purified ben EXAMPLE 4 zoic acids of two different qualities whose total amount andrelative proportions can be adjusted to fit commercial A genes Water of crude 136112910 9 requirements by changing the quantity of water used and was run at 99 as described E m which the temperatures at which the Steps of the process are the filtrate obtained from the benzoic acid separation from operated. The purity of each benzoic acid product may the aqileous Phzlse was water used i Subsequfint Similarly be varid as required extraction. As shown in Table 2, the purity of the ben- Thus, the first step may be carried out at a tempera- Z016 Field Products was not adversely a ed y thls mm from about up to about C Since technique and there was no build-up of impurities in the liquid water and liquid benzoic acid become miscible in waterall proportions at 117.2 C. Preferably this first sepa- Table 2 ration is brought about at 113116 C. A relatively high temperature in this step makes possible the extrac- Product P F- Mot-her liquor cent benzoic p.p.rn. tion of a larger quantity of benzoic acid from the crude acid impurities organic layer and the separation of more purified benzoic acid in the second step. Conversely, lower temperatures iltttzer firstegtratcrtio .7 50 produce purified benzoic acid less in amount but of 9 BX somewhat higher quality. Similarly, the distribution of tiiiii tiih thiitihjj: 3333 w 50 benzoic acid between the organic and aqueous layers formed in the second step of the process depends upon Not measured. variation of the temperature limits of 95-100 C. in this Example 5 illustrates batchwise operation of the Proc P- ess when carried out with a first phase separation at super- The water extraction step in either mode of running heated Water temperatures the process is carried out in the presence of sufiicient turbulence or agitation to attain substantial equilibrium 35 EXAMPLE 5 between the {W0 liquid layers produced- The impure benzoic acid used in this example was Examples illustrate the results Obtained y Water similar to that used in Examples 1-4. A stainless steel extraction of crude b nz i a d at cylinder of 500 ml. capacity and equipped with a valve EXAMPLES at e ach end was charged with 69 g. of impure benzoic acid and 261 g. of water and heated to 115 C. with oc- A crude benzoic acld Product Obtallled y ytic casional shaking. The lower organic layer amounting air oxidation of toluene was subjected to preliminary to 55 was drawn Off and the upper aqueous layer purification to remove catalyst residues, unreacted toluwas t f d to a Separatm-y funnel and its temperaone, and the bulk of oxidation impurities. The resulting ture was adjusted to A Second Separation into material Was benzoic acid of pel'callt P y by aqueous and organic liquid layers took place and the gas Chromatographic analysis- Portions of thls Product organic layer was drawn off. This second organic layer were mixed with different amounts of water calculated i h d 245 g, d it was f d to be a solution f Water to give two-Phase liquid Systems at having q in liquid benzoic acid containing 72.9 percent by weight 0115 layer to btiIlZOic acid layer Weight ratios of 81/1, of benzoic acid. The solid benzoic acid obtained by and respectively- These three miXtuIeS 50 cooling and drying the second organic layer was of 99.5 were heated to in Steam-jacketed p y percent purity. The second aqueous layer was cooled H618 With Occasional Shaking The heavy P COII- to about 25 C., thereby causing separation of pure white {aiming bout 70 percent benzoic acid, were separated crystalline benzoic acid. The dried product weighed 14 and the light or aqueous phases were then rapidly cooled r g. and was of exceptional purity, assaying essentially 100 to 25 C. Crystalline benzoic acid precipitated from 59 percent benzoic acid on adry basis. each and these products were separated by filtration, Thi process can also be operated in a continuous mandried, and analyzed. The benzoic acid contained in the nor wherein the first step is a liquid-liquid countercurheavy phases was similarly separated and analyzed by rent extraction carried out in a conventional extraction gas chromatography. The results of these experiments column or tower at the appropriate temperature. The are listed in Table 1. Impurities are given in parts per 60 design of the column and the feed rates of the crude million. benzoic acid and hot water are such that substantial equi- Table 1 Phase ratio 8.1/1 Phase ratio 22.1/1 Phase ratio 47.8/1 Starting Impurity Material Lt hase H hase Lt hase Hv h Lt hase H be e 1322 cid i'a m bz'. gcid bi'a iii bz: em dia in Benzaldehyde 50 280 50 420 50 380 Benztiestersnn 190 250 75 350 75 430 Benzyl alcohol s00 20 960 20 i, 680 20 2, 210 Aromatic Hydrocarbons- 12,150 280 18,700 710 31,500 1, 280 t 50, 860 Unknowns.. 950 55 1, 280 60 2, 210 Total Impur1ties,pereent 1.4 0.05 2. 15 0.09 3.62 O. 15 5.75 g

librium is reached in operation of the process and a stable interface between the liquid phases is maintained, otherwise the conditions of the process are as set forth previously. The molten benzoic acid stream is introduced at or near the top of the extraction column and the extracted rafiinate is continuously drawn off from the bottom of the column whence it may be recycled to the extraction process or returned to the main phenol process feed stream. Hot water is fed in near the bottom of the column and the aqueous extract flows continuously from an outlet near the top. Where the extraction is run at 95-100 C., the aqueous extract is piped to a crystallizer wherein it is cooled and the resulting slurry is transferred to filtration means. Where the extraction is accomplished at 110-117 C., the aqueous extract flows continuously from an outlet near the top to a separator wherein the second stage of the process is operated at 95-100 C. and whence the second aqueous layer and the second organic layer are separated and worked up as previously described. The aqueous filtrate from the final crystallization of benzoic acid in either mode of operation is advantageously recycled to the extraction column. The above description of continuous operation of the process is general in nature and minor variations in operating techniques conventional in the continuous extraction art may be introduced as desired to fit individual situations. For example, the extraction tower or column may be replaced by a series of interconnected vessels serving as combined mixing and settling tanks operating at the same or different temperatures, wherein there is countercurrent flow of the aqueous and organic layers, thereby accomplishing the same results obtained from the extraction in an extraction column as described above. Such multistage extraction can be designed to fit various purity and yield requirements, whatever form the extraction apparatus takes.

Other conventional variations can be introduced into the purification process, for example, solvent extraction of the aqueous extract to reduce further the impurity content.

I claim:

1. A process for purifying crude benzoic acid obtained by the catalytic oxidation of toluene with an oxygen-containing gas, which process comprises contacting said crude benzoic acid in the molten state with 06-14 parts by weight of water per part of crude benzoic acid at a temperature of about 95 C. to about 117 C., thereby forming two liquid benzoic acid-containing phases in substantial equilibrium, said liquid phases consisting of a light aqueous phase and a heavy organic phase, separating said phases, cooling said aqueous phase, thereby causing precipitation therefrom of purified benzoic acid, and separating said purified benzoic acid.

2. The process of claim 1 wherein the crude benzoic acid is continuously contacted with water in a countercurrent extraction column.

3. A process for purifying crude benzoic acid obtained by the catalytic oxidation of toluene with an oxygencontaining gas, which process comprises contacting said crude benzoic acid in the molten state at -100 C. with about 06-14 parts by weight of water, thereby forming two liquid phases in substantial equilibrium, said phases consisting of a light aqueous phase and a heavy organic phase, separating said phases, cooling said aqueous phase to a temperature below 94.6 C., thereby causing precipitation therefrom of purified benzoic acid, and separating said purified benzoic acid.

4. The process of claim 3 wherein 0.7-7 parts by weight of water are employed per part of crude benzoic acid.

5. The process of claim 3 wherein the crude benzoic acid is continuously contacted with water in a countercurrent extraction column.

6. A process for purifying crude benzoic acid obtained by the catalytic oxidation of toluene with an oxygencontaining gas, which process comprises contacting said benzoic acid in the molten state with about 0.7 to about 7 parts by weight of water at about 110 C. to about 117 C. in an extraction zone under sufiicient pressure to maintain the water in the liquid state, thereby forming two liquid phases in substantial equilibrium, said phases consisting of a first aqueous phase and a first organic phase, separating said phases, cooling said first aqueous phase to a temperature of about 95 C. to about C., thereby causing the formation of two liquid phases consisting of a second aqueous phase and a second organic phase, separating said second phases, and cooling said second aqueous phase to a temperature below 94.6" C., thereby causing crystallization therefrom of benzoic acid.

7. The process of claim 6 wherein 0.8-5 parts of water are employed per part of benzoic acid.

8. The process of claim 6 wherein the crude benzoic acid is continuously contacted with water in a countercurrent extraction column.

References Cited by the Examiner UNITED STATES PATENTS 2,189,726 2/1940 Conover 260525 2,252,117 8/1941 Conover 260525 2,572,575 10/1951 Shafer et al. 260-525 2,963,509 12/1960 Barker et al. 260-524 FOREIGN PATENTS 768,273 2/ 1951 Great Britain.

OTHER REFERENCES Handbook of Chemistry and Physics, pp. 750-1, 772-3, and 788-9 (35th ed., 1953-4).

LORRAINE A. WEINBERGER, Primary Examiner. LEON ZITVER, Examiner. 

1. A PROCESS FOR PURIFYING CRUDE BENZOIC ACID OBTAINED BY THE CATALYTIC OXIDATION OF TOLUENE WITH AN OXYGEN-CONTAINING GAS, WHICH PROCESS COMPRISES CONTACTING SAID CRUDE BENZOIC ACID IN THE MOLTEN STATE WITH 0.5-14 PARTS BY WEIGHT OF WATER PER PART OF CRUDE BENZOIC ACID AT A TEMPERATURE OF ABOUT 95*C. TO ABOUT 117*C., THEREBY FORMING TWO LIQUID BENZOIC ACID-CONTAINING PHASES IN SUBSTANTIAL EQUILIBRIUM, SAID LIQUID PHASES CONSISTING OF A LIGHT AQUEOUS PHASE AND A HEAVY ORGANIC PHASE, SEPARATING SAID PHASES, COOLING SAID AQUEOUS PHASE, THEREBY CAUSING PRECIPITATION THEREFROM OF PURIFIED BENZOIC ACID, AND SEPARATING SAID PURIFIED BENZOIC ACID. 